I've been able to download several GB of files, via direct and torrent, with a router uptime of several days.
No issues with a LAN download. No issues with LAN torrents. No issues with Xbox downloading multiple GB game demos, which is effectively LAN for me.
Only 1 reboot so far caused by my xbox360 trying to stream a movie from one of my LAN computers. This is effectively LAN to LAN connection since the DGL sees the connection coming in via Ethernet.
It sounds like numerous people are experiencing issues with their router with wireless enabled. There have been multiple reports of wireless causing network errors to show up, and I can only imagine heavy downloads worsening this effect. I don't have either Wireless or UPNP enabled, and this might account for my stability.